This invention relates to an improvement in a method for the preparation of melamine by the conversion of urea and/or thermal decomposition products thereof wherein the melamine synthesis reactor effluent is cooled with an aqueous medium to form a aqueous melamine solution or suspension wherefrom solid product melamine is eventually recovered. The melamine synthesis reactor effluent additionally contains ammonia and carbon dioxide which are also in part dissolved in the aqueous medium used to quench the melamine effluent. Such a method for the preparation of melamine is known from, for instance, U.S. Pat. No. 3,711,479.
A number of difficulties arise in the preparation of melamine, particularly in the processing of the above-noted melamine solutions or suspensions which additionally contain ammonium carbamate. Aqueous solutions of ammonia and carbon dioxide (hereinafter referred to as aqueous ammonium carbamate solutions) are known to have a very corrosive effect on several types of stainless steel, particularly at elevated temperatures.
The present invention specifically relates to an improved process for the preparation of melamine utilizing as a material of construction a particular chromium-nickel stainless steel alloy having a ferritic-austenitic structure, in order to combat the severe corrosion and errosion conditions uniquely encountered in the preparation of melamine.
Extensive research has been conducted into the suitability of various alloys as materials of construction for surfaces coming into contact with aqueous ammonium carbamate solutions. This research, however, has particularly centered around ammonium carbamate solutions formed in connection with the preparation of urea. For example, a survey of corrosion rates of several chromium-nickel steels subjected to aqueous ammonium carbamate solutions in urea synthesis processes is set forth in an article by R. deJonge et al entitled "Corrosion and Corrosion Prevention in Stainless Steel Urea Plants", Chem. Age. of India 26 pp. 249-260 (1975).
Chromium-nickel alloys having a duplex ferritic-austenitic structure of the type utilized herein have been known and utilized in chemical processing for a number of years. See, for example, German Auslegeschrift No. 1,608,236. Moreover, such alloys have been suggested as a material of construction for apparatus utilized in preparation of urea. Thus, British Pat. No. 1,153,107 describes a method for preparing urea wherein one or more surfaces that come into contact with ammonium carbamate consist of a steel alloy having at least 20 percent by weight chromium, 1-7 percent by weight nickel, and 1-4 percent by weight molybdenum.
The use of ferritic-austenitic steels containing 25 percent by weight chromium, from 1 to 6 percent by weight nickel, and from 1 to 3 percent by weight molybdenum are also noted in British Pat. No. 1,333,544 as being suitable for use in urea synthesis processes. Even with this material, however, it is recommended that passivating oxygen be utilized. This reference then goes on to teach that the disadvantages inherent in the use of passivating oxygen can be avoided in urea processing by utilizing on surfaces coming into contact with ammonium-carbamate a nickel-free ferritic steel consisting essentially of at least 25 percent by weight of chromium, and from 0 to 3 percent by weight molybdenum, with a combined carbon plus nitrogen content of not more than 0.035 percent by weight. However, such nickel-free chromium-iron alloys have the disadvantage that special measures must be taken to preserve their corrosion resistance when they are welded, as is necessary in the fabrication of processing equipment.
Although ammonium carbamate is believed to be the major cause of equipment corrosion in both the preparation of melamine and the preparation of urea, it has been found that a material suitable for the preparation of urea is not, as a matter of course, suitable for the preparation of melamine. Conversly, those materials found suitable for the preparation of melamine are not necessarily also suitable for the preparation of urea. Extensive research has been conducted in an attempt to explain why this is so, but without success.
A clear distinction between the two processes lies in the fact that in the preparation of urea it is possible to reduce corrosion by adding a minor amount of oxygen to passivate the stainless steel. In general, it is essential when using chromium-nickel steels that oxygen should be added. It is not feasible, however, to add oxygen in the preparation of melamine as the presence of oxygen interferes with the preparation of melamine. Nevertheless, this difference does not account for the difference in corrosion performance found with various of alloys when used as a material of construction for the preparation of urea, on one hand, and for the preparation of melamine, on the other.
An object of the present invention is to provide a method for the preparation of melamine, as described above, wherein the corrosion of the processing equipment caused by aqueous solutions of ammonium carbamate, as well as errosion due to the presence of solid matter in the liquid streams, is reduced to a minimum.